Collisional Excitation Cross Sections Research Articles

NIFS Atomic Numerical Databases

AbstractWe have compiled the atomic and molecular numerical databases which are available through internet. The databases provide basic atomic data, such as collisional ionization and excitation cross sections, which are important for modeling and diagnosing astrophysical plasmas.

Selective hyperfine excitation of N2H+ by He: potential energy surface, cross sections, and propensity rules.

We present potential energy surfaces for the He-N2H+ system adiabatically corrected for the zero-point motion along the intermolecular stretching vibrations v1 = 0 and v1 = 1. The potentials are extended to shorter He-N2H+ separations which makes them useful for scattering calculations. Close coupling calculations of the spinless S matrices for the rotational excitation of N2H+ by He are presented, and recoupling techniques to obtain collisional excitation cross sections between the N2H+ hyperfine levels are used. The propensity rules between hyperfine levels are investigated for the case where two nuclear spins are involved. It is found that the only well defined propensity rule is DeltaF = DeltaF1 = Deltaj and that calculations are required in order to obtain the relative intensities of the two-spin hyperfine cross sections.

Accurate cross sections for excitation of resonance transitions in atomic oxygen

Electron collision excitation cross sections for the resonance 2p43P–2p33s3So, 2p43P–2p33d3Do, 2p43P–2p33s3Do, 2p43P–2p33s3Po, and 2p43P–2s2p53Po transitions have been calculated by using the R matrix with a pseudostates approach for incident electron energies from near threshold to 100 eV. The excitation of these transitions gives rise to strong atomic oxygen emission features at 1304, 1027, 989, 878, and 792 Å in the spectra of several planetary atmospheres. We included 22 spectroscopic bound and autoionizing states and 30 pseudostates in the close‐coupling expansion. The target wave functions are chosen to properly account for the important correlation and relaxation effects. The effect of coupling to the continuum is included through the use of pseudostates. The contribution of the ionization continuum is significant for resonance transitions. Measured absolute direct excitation cross sections of O I are reported by experimental groups from the Jet Propulsion Laboratory and Johns Hopkins University. Good agreement is noted for the 2p43P–2p33s3So transition (λ1304 Å) with measured cross sections from both groups that agree well with each other. There is disagreement between experiments for other transitions. Our results support the measured cross sections from the Johns Hopkins University for the 2p43P–2p33d3Do and 2p43P–2p33s3Do transitions, while for the 2p43P–2p33s3Po transition the agreement is switched to the measured cross sections from the Jet Propulsion Laboratory.

Doppler spectroscopy of hydrogen and deuterium balmer alpha line in an abnormal glow discharge

The results of hydrogen and deuterium Balmer alpha line shapes and line intensities study in an abnormal glow discharge are reported and analyzed. The Doppler shifts along line wings are used to determine energies of excited hydrogen and deuterium atoms. For 12 different cathodes, intensity and shape of line wings are examined and dependence upon cathode material is determined. Tentative explanation of line wings intensity dependence is related to the sputtering of cathode material and back-scattering coefficients of incident hydrogen or deuterium ions and atoms from cathode surface. The influence of the light reflected on a cathode surface to the line shape measurements along discharge axis is considered. In hydrogen, deuterium, and Ar+3%H/sub 2/ discharges, basic mechanisms of fast hydrogen generation and excitation are studied. The shape and intensities of the H/sub /spl alpha// line profiles in pure hydrogen and in argon-hydrogen mixture may be correlated with hydrogen atom-carrier gas collision excitation cross sections. In order to assess the importance of reflected fast hydrogen atoms back scattered from the cathode surface, for the Balmer line shape formation, a simulation program is used. The results are in a qualitative agreement with Balmer line shapes observations.

Atomic data for the K-vacancy states of Fe XXIV

As part of a project to compute improved atomic data for the spectral modeling of iron K lines, we report extensive calculations and comparisons of atomic data for K-vacancy states in Fe XXIV. The data sets include: (i) energy levels, line wavelengths, radiative and Auger rates; (ii) inner-shell electron impact excitation rates and (iii) fine structure inner-shell photoionization cross sections. The calculations of energy levels and radiative and Auger rates have involved a detailed study of orbital representations, core relaxation, configuration interaction, relativistic corrections, cancellation effects and semi-empirical corrections. It is shown that a formal treatment of the Breit interaction is essential to render the important magnetic correlations that take part in the decay pathways of this ion. As a result, the accuracy of the present A-values is firmly ranked at better than 10% while that of the Auger rates at only 15%. The calculations of collisional excitation and photoionization cross sections take into account the effects of radiation and spectator Auger dampings. In the former, these effects cause significant attenuation of resonances leading to a good agreement with a simpler method where resonances are excluded. In the latter, resonances converging to the K threshold display symmetric profiles of constant width that causes edge smearing.

Charge dependence of electron capture and excitation cross sections in collisions of protons with closed-shellNa20+qq+clusters

We have evaluated electron capture and excitation cross sections in collisions of ${\mathrm{H}}^{+}$ with isoelectronic ${\mathrm{Na}}_{20+q}^{q+}$ clusters $(q=\ensuremath{-}1,0,+1)$ in the impact energy range 40\char21{}500 eV. The theoretical method includes the many-electron aspect of the problem and makes use of realistic cluster potentials obtained with density-functional theory and a spherical jellium model. As expected, electron capture decreases monotonically with cluster charge, but the corresponding mechanism does not follow a common pattern. Excitation cross sections are very similar in the three cases investigated here.

Semiclassical inelastic electron-ion collisional excitations in nonideal plasmas

Semiclassical electron-hydrogenic ion collisional excitation cross sections for 1s→2s and 1s→2p0 in nonideal plasmas are obtained. An effective pseudopotential model taking into account the plasma screening and collective effects is applied to describe the electron-ion interaction potential in a classical nonideal plasma. The semiclassical straight-line trajectory approximation is applied to the motion of the projectile electron in order to investigate the variation of the total inelastic scattering cross section as a function of the projectile energy, nonideal plasma parameter, and Debye length. It is found that the transition probabilities in ideal plasmas described by the Debye–Hückel potential model are greater than those in nonideal plasmas described by the pseudopotential model. The maximum position of the transition probability gets closer to the target core as the projectile energy increases. It is also found that the scaled cross section decreases when the nonideal plasma parameter (γ) is increased, i.e., the nonideality of plasmas reduces the electron-ion collision cross section.

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Experimental and theoretical collisional excitation cross sections are reported for the 3s23p2 3P → 3s3p3 5So spin-forbidden transition in S2+. The transition energies (wavelengths) are 7.171 eV (1728.9 A, J = 2 → 2) and 7.237 eV (1713.1 A, J = 2 → 1). In the experimental approach, use is made of the electron energy-loss method with merged electron and ion beams. The metastable fraction in the S2+ beam is assessed and minimized. A novel electronic aperture is used to limit the trajectories of elastically scattered electrons into the trochoidal electrostatic analyzer. Theoretically, the -matrix method in a 17 state and a 27 state calculation is used to obtain cross sections from threshold to 15.0 eV. Good agreement is found between experiment and the results of the 27 state theory. The presence of two strong resonances near threshold (7.5-9.0 eV) and a third resonance above threshold (11.5 eV) is verified by experiment.

Empirical Regularities in the Behavior of the Dissociation Excitation Cross Sections in Collisions of Electrons with Cesium–Halide Molecules

The behavior of the constants Ai. and αi that characterize the dependence of the dissociation excitation cross sections on the main quantum number of the upper energy level is analyzed for spectral series of the cesium atom in collisions of electrons with cesium–halide molecules. In addition to a similarity, some deviations from analogous regularities established previously for the halides of other alkali metals are also found.

Thin-film electroluminescent devices excited by a linearly rising voltage

The average and instantaneous luminances of a thin-film electroluminescent device (TFELD) are determined as functions of the voltage rise time by solving kinetic equations for the concentration of excited emission centers in the electroluminescent layer of the device. It is shown theoretically and experimentally that the dependences of the average and peak luminances, the external and internal quantum yield, the energy yield, and the luminous efficacy as functions of the voltage rise time all have a maximum, and the position of that maximum depends on the frequency of the driving voltage. The calculated and experimental dependences make it possible to determine the main parameters of the electroluminescence process: the collisional excitation cross section for the emission centers, the concentration of emission centers, and the transition probability of the emission centers to an excited state, as well as the radiative and nonradiative recombination probabilities of these and other centers.

Excitation autoionization rates from ground and excited levels in Li-likeAr15+to S-likeAr2+

The rate coefficients for excitation autoionization (EA) from the levels of the ground and first excited configuration in nearly all argon ions (Li-like Ar${}^{15+}$ to S-like Ar${}^{2+})$ have been computed. Collisional excitation cross sections are computed in the distorted-wave approximation and integrated over a Maxwellian distribution of free-electron energies. Radiative and autoionizing transition probabilities are computed using the fully relativistic, intermediate coupling code RELAC. For each ion, the rates of EA are compared to semi-empirical calculations of the direct ionization rate. For charge states with few $M$-shell electrons in the ground configuration (Na-, Mg-, and Al-like), EA can significantly enhance the direct ionization rate coefficients. In the first excited configuration of each ion considered, the EA rate coefficient enhances the direct ionization rate coefficient only at low temperatures.

Collisional excitation and destruction of metastable states in a stored ion beam

A laser probing technique has been applied to a stored ion beam of ${\mathrm{Xe}}^{+}$ to study collisional destruction of metastable states as well as collisional excitation by the residual gas in the storage ring. The results are of great importance for the evaluation of lifetime measurements of metastable states in storage rings. For the states studied in ${\mathrm{Xe}}^{+},$ it was found that neither of the two processes caused systematic errors in the lifetime determination. It is also shown that the method can yield partial cross sections for collisional excitation.

Electron Excitation Cross Sections for the SiiTransitions 3s23p34So→ 3s23p32Do,2Po, and 3s3p44P

Experimental and theoretical collisional excitation cross sections are reported for the transitions 3s23p34So → 3s23p32Do,2Po, and 3s3p44P in S II. The transition wavelengths (energies) are 6716 A (1.85 eV), 4069 A (3.05 eV), and 1256 A (9.87 eV), respectively. In the experiments, use is made of the energy-loss merged-beams method. The metastable fraction of the S II beam was assessed and minimized. The contribution of elastically scattered electrons was reduced by the use of a lowered solenoidal magnetic field and a modulated radio-frequency voltage on the analyzing plates and by retarding grids to reject the elastically scattered electrons with larger Larmor radii. For each transition, comparisons are made among experiments, the new 19 state -matrix calculation, and three other close-coupling calculations.

Status of the multiply-charged ion research facility at JPL

A new research facility for the study of interactions of multiply-charged ions has recently been completed at the Jet Propulsion Laboratory. A 1.2 tesla/14.0 GHz Caprice-type electron-cyclotron-resonance (ECR) ion source is used to produce highly-charged ions. The JPL facility can deliver ions into three experimental stations. Two are currently in operation for (a) measurement of absolute electron collisional excitation cross sections, and (b) measurement of lifetimes and f-values of metastable levels.

Spectrum of plasma containing Ne- and Na-like ions: consistent account for Rydberg and autoionizing Rydberg series in balance equations

This is a theoretical study of argon plasma under conditions, where Ne- and Na-like ionization stages are dominant. Balance equations are generalized to treat 37 3l states of Ne-like ion and 37 adjacent series of Rydberg states of Na-like ion simultaneously. This allows us to include in the kinetics a diffusion-like motion of the state of the system "Ne-like ion plus one electron" through the multitude of excited levels spread over an energy region of 50 eV. The populations Ni - i (i = (1, ..., 37)) of the Ne-like ion states are introduced explicitly; those of adjacent 37 Rydberg series are accounted for through continuous functions Ni(ε). These functions describe the population distribution within each Rydberg series dependent on the Rydberg electron enegy ε. The elementary processes of the collisionalradiative model connecting all Ne- and Na-like states, as well as processes of redistribution of populations inside each Rydberg series, are accounted for. The rate coefficients for all processes within the Ne-like residue have been calculated previously, using a detailed many-body relativistic theory. The dielectric capture cross sections and autoionization probabilities are presented as analytical continuation of the collisional excitation cross sections. The excited-excited states transitions are included. The rest of the processes are treated in a simple semiclassical approximation. The Lotz formula is generalized by unambiguous analytical continuation to cover the case of bound-bound transitions between Rydberg states of Na-like ions. The radiation reabsorption in a long plasma cylinder is included through Biberman-Holstein coefficients for all transitions. The inclusion of Na-like states, accounting for diffusion-like processes, increases the population inversion for the "lasing candidates" by at least a factor of two for a wide range of plasma conditions. This is important for the ionization equilibrium too. Besides, the functions Ni(ε) bear diagnostic information. Detailed calculations have been done for the homogeneous steady-state Maxwellian plasma. The role of transient processes in the population inversion creation is under discussion.

The influence of metastables on the plasma-induced emission in a helium rf discharge

Space- and time-resolved emission spectroscopy is commonly used to investigate heating mechanisms in rf discharges (plane parallel plate configuration). The metastable density in these discharges is several orders of magnitude lower than the density of the ground-state atoms; however, the electron collision excitation cross sections of some helium levels out of the metastable levels exhibit values which are several orders of magnitude larger and have much lower thresholds than those for the ground state. It is therefore questionable whether stepwise excitation from metastable levels can be neglected in the interpretation of the spatiotemporal plasma-induced emission of rf discharges. To our knowledge only the excitation of ground-state atoms has been taken into account in model calculations to this date. With respect to the energy balance of such a discharge, this assumption is probably correct. However, with respect to the emission of light, our computational results show that some specific levels are excited significantly by electron collisions with metastables. We will show in this contribution, on the basis of our measurements, that this effect is responsible for the observed emission profiles of some lines. Consequently, the interpretation of the emission profiles in helium rf discharges has to take account of the role of metastables.

A probe of dynamical models using functional sensitivity densities with application to He++Ne(2p 6)?He++Ne(2p 53s) and Li+I?Li++I?

The functional sensitivity densities δ ln σ12(E)/δ lnVii(R) for He++Ne(2p6)→He++Ne(2p53s) reveal that the collisional excitation cross section σ12(E) is insensitive to the additional diabatic curveV33 included in some models. The negligible sensitivity of σ12(E) toV33 offers a quantitative validation of the more popular two state model for collisional excitation of Ne by He+. The sensitivity profiles for the collisional ionization Li+I→Li++I− modeled by crossing diabatic curvesV11 (covalent) andV22 (ionic) shows that the ionization cross section does not depend on inner crossings even when these stem from large distortions in the underlying potential energy curves. The lack of sensitivity to inner crossings establishes the predominant role of the outermost crossing in triggering nonadiabatic transitions.

Wavefunction overlap effects in the collisional excitation of NH3 by slow molecular ions

Collisional excitation of ammonia from the ground 1A1 electronic state to various optically-allowed and dipole-forbidden states has been studied in 1.8 keV collisions with H2+, N2+, O2+ and CO2+ projectiles by energy loss spectrometry using a high-resolution ion translational energy spectrometer. The collisional excitation cross section is found to be sensitively dependent on whether the projectile ion is of Sigma or Pi symmetry. This dependence is interpreted as a manifestation of wavefunction overlap effects involving the highest occupied molecular orbitals of the target and projectile ions. Results of concomitant theoretical calculations of overlap integrals for different orientations of each projectile-target system provide support for the wavefunction overlap picture.

Collisional processes involved in the population kinetics of semiconductor quantum-dot lasers.

We present an application to quantum dots of formulas from atomic physics for the rates of collisional ionization (binary encounter model) and dipole allowed collisional excitation (Van Regemorter and classical path models) occurring via the Coulomb interaction. The Van Regemorter, classical path, and binary encounter cross sections, which are known to be very successful in modeling atomic collisions, are shown to have accuracies comparable to (and are much easier to use than) the methods currently being used to study collisional rates in bulk excitons. Collisional excitation and ionization cross sections for a 150-\AA{}-radius InSb quantum dot surrounded by CdTe barriers are shown to be much smaller than the geometric area \ensuremath{\pi}${\mathit{R}}^{2}$, where R is the quantum-dot radius. The reason that these cross sections are so small is that the Coulomb energy associated with the collision is typically much smaller than the collisional energy exchange. Explicit formulas are given for quantum-dot intraband oscillator strengths.

Cross sections for electron scattering by atomic potassium.

Electron elastic and collisional excitation cross sections from the ground state of potassium are calculated using the noniterative integral-equation method of Henry, Rountree, and Smith [Comput. Phys. Commun. 23, 233 (1981)] in the electron energy range 4\ensuremath{\le}E\ensuremath{\le}200 eV. Configuration-interaction target wave functions that take account of correlation and polarization effects are used to represent the ground state and the six lowest excited states 4p $^{2}$P\ifmmode^\circ\else\textdegree\fi{}, 5s $^{2}$S, 3d $^{2}$D, 5p $^{2}$P\ifmmode^\circ\else\textdegree\fi{}, 4d $^{2}$D, and 6s $^{2}$S. Elastic and discrete excitation cross sections are obtained in a seven-state close-coupling (7CC) approximation. The 7CC elastic and excitation cross sections are compared and contrasted. Near threshold the elastic cross section dominates the resonance, 4s $^{2}$S\ensuremath{\rightarrow}4p $^{2}$P\ifmmode^\circ\else\textdegree\fi{}, and the sum of the other remaining excitation cross sections. Comparison of our total cross sections with some available experimental and theoretical data is also effected. The discrepancy between the recent measurement of the total cross section by Kwan et al. [Phys. Rev. A 44, 1620 (1991)] on the one hand and other measurements near threshold on the other hand is explained.